The present invention relates to a pilot injection system used in a fuel injection pump, and more specifically, to a pilot injection system in which a stack of piezoelectric elements is used as an actuator.
In general, diesel engines of automobiles, during idle running, produce higher levels of noise and vibration than gasoline engines. It is well-known, however, that the levels of noise and vibration of diesel engines can be lowered by performing pilot injection before the main injection of fuel into the combustion chambers of the engine.
Therefore, fuel injection pumps for diesel engines are commonly expected to be able to perform pilot injection, as well as main injection. One such fuel injection pump is stated in Japanese Patent Disclosure No. 59-18249. This prior art pump has a pump chamber which is supplied with fuel. The fuel in the pump chamber is pressurized by a plunger. The pressurized fuel is injected through injection nozzles into the combustion chambers of the engine. Also, the pump has a pressure chamber which is connected to the pump chamber. The pressure chamber is defined by a piston, which is coupled with a stack of piezoelectric elements, for use as an actuator. The stack tends to extend or contract, depending on the value of the voltage applied thereto. In the fuel injection pump of this type, after the pressurized fuel in the pump chamber starts to be injected into the combustion chambers of the engine, the fuel pressure in the pump chamber, which is connected to the pressure chamber, is lowered instantaneously when the piston is moved by contraction of the stack, thereby increasing the capacity of the pressure chamber. As a result, the internal fuel pressure of the pressure chamber temporarily becomes lower than the opening pressure of the injection nozzles. Thus, the fuel injection into the combustion chambers is stopped for a while, permitting the pilot injection.
In the fuel injection pump having the pilot injection system of the aforementioned construction, it is evident that the change of capacity of the pump chamber, which is conducive to the pilot injection, depends on the degree of contraction of the stack. However, the stack contracts to only a relatively low degree. With use of the conventional pilot injection system, therefore, the pilot injection cannot be separated definitely from the main injection subsequent thereto. Thus, the desired pilot injection cannot be ensured with ease. On the other hand, the stack can be contracted to the desired degree if it is increased in size. In such a case, however, the injection system itself must be large-sized.
In the prior art pilot injection system, moreover, even while the engine is operating at high speed, without requiring pilot injection, the high fuel pressure in the pump chamber is directly applied through the piston to the stack. Therefore, the piezoelectric elements of the stack are liable to be damaged after prolonged use.